Bug 1360144 - make stroke-{*} animatable for stylo. r=boris

This part includes making stroke-linecap, stroke-linejoin, stroke-miterlimit,
stroke-opacity, and stroke-dasharray animatable.

For properties that already implemented Interpolate trait and clone() for
glue code, we can just make them animatable by replacing the animation_value_type
with proper type name. So, set animation_value_type to 'discrete' for
stroke-linecap and stroke-linejoin. Set animation_value_type to 'ComputedValue'
for stroke-miterlimit and stroke-opacity.

As to stroke-dasharray, we need to implement Interpolate trait and glue codes
for it.

components/style/properties/helpers/animated_properties.mako.rs

@@ -621,6 +621,8 @@ pub trait Interpolate: Sized {
 /// https://drafts.csswg.org/css-transitions/#animtype-repeatable-list
 pub trait RepeatableListInterpolate: Interpolate {}
 
+impl RepeatableListInterpolate for Either<f32, LengthOrPercentage> {}
+
 impl<T: RepeatableListInterpolate> Interpolate for SmallVec<[T; 1]> {
     fn interpolate(&self, other: &Self, progress: f64) -> Result<Self, ()> {
         use num_integer::lcm;